The present invention relates to a light-receiving semiconductor device including light responsive elements such as split photodiodes used in an optical pickup, and more particularly, to a structure for reducing light surface reflectance in the proximity of a dividing region.
Split photodiodes are used for detecting a light signal in, for example, an optical pickup. FIG. 9 shows a sectional view of conventional two-split photodiodes. Two sets of such 2-split photodiodes are provided in parallel to obtain a 4-split photodiode.
According to a method of manufacturing the split photodiodes shown in FIG. 9, a silicon substrate including a plurality of N-type silicon layers 1 and 2 is provided. In general, an N.sup.- type epitaxial layer 2 is grown on an N.sup.+ type silicon layer 1. Alternatively, N.sup.+ diffusion layer 1 may be formed from the bottom of an N.sup.- type silicon layer 2. A pattern of a SiO.sub.2 film 7 is formed on N.sup.- type silicon layer 2. Using this pattern of SiO.sub.2 film 7 as a mask, a plurality of P.sup.+ type anode layers 9 are formed at the surface of N.sup.- type silicon layer 2. Anode layer 9 is covered by an anti-light-reflecting film of a silicon nitride film 8.
FIG. 10 shows a plan view of 6-split photodiodes used in a conventional optical pickup. Referring to FIG. 10, the middle four photodiodes 10a, 10b, 10c and 10d are used for detecting a focus signal. The two diodes 10e and 10f at both sides are used for detecting a tracking signal.
FIG. 11 shows a plan view of 5-split photodiodes used in an optical pickup employing a hologram. The left middle two photodiodes 11a and 11b are used for detecting a focus signal and a reproduced signal. The right middle photodiode 11c is used for detecting a reproduced signal. The two outside photodiodes 11d and 11e are used for detecting a tracking signal.
FIG. 12 schematically shows the arrangement of the main components in an optical pickup employing a hologram. Referring to FIG. 12, light emitted from a laser diode 13 sequentially passes through a hologram 14, a collimator lens 15, and an objective lens 16 to be focused on a record surface of an optical disk 17. Light reflected from optical disk 17 passes through objective lens 16, collimator lens 15, and hologram 14 to be focused on split photodiodes 18 such as those shown in FIG. 11.
Referring to FIGS. 13A, 13B and 13C, astigmatism is described as an example of a method of detecting focus error using 4-split photodiodes. The 4-split photodiodes include a pair of photodiodes 13a and 13d located adjacent in one diagonal direction, and another pair of photodiodes 13b and 13c adjacent in another diagonal direction. These four photodiodes are separated from each other by a dividing region D. A light beam projected on the 4-split photodiodes is represented as a spot 20. The shape of light spot 20 varies depending on focus error.
FIG. 13A shows light spot 20 on split photodiodes when the focus point is properly on the record surface of the optical disk. FIG. 13B shows light spot 20 on 4-split photodiodes when the record surface of the optical disk is closer than the focus point. FIG. 13C shows a light spot 20 on 4-split photodiodes when the record surface of the optical disk is farther than the focus point.
In general, the outputs of the first pair of photodiodes 13a and 13d adjacent in a first diagonal direction are summed. Similarly, the outputs of the second pair of photodiodes 13b and 13c adjacent in a second diagonal direction are summed. The difference between the summed outputs of the two pairs of photodiodes is detected as a focus error signal.
More specifically, a focus error output signal S is expressed by the following equation: EQU S={(output signal of photodiode 13a)+(output signal of photodiode 13d)}-{(output signal of photodiode 13b)+(output signal of photodiode 13c)}(1)
If S=0, the record surface of the optical disk is at the proper focus point. If S&gt;0, the record surface of the optical disk is closer than the focus point. If S&lt;0, the record surface of the optical disk is farther than the focus point.
It is appreciated from light spot 20 in FIGS. 13A-13C that a major portion of the signal optical beam is projected onto dividing region D.
A major portion of the light-receiving plane of P.sup.+ type anode region 9 is covered with an anti-light-reflecting film of a silicon nitride film as shown in FIG. 9. There is an PN junction between dividing region D and anode region 9. If a silicon nitride film is formed on the surface of a semiconductor across a PN junction, surface leak current across that PN junction increases. Therefore, a SiO.sub.2 film 7 is left on the PN junction between dividing region D and its relevant sides. It is difficult to reduce the width of dividing region D and SiO.sub.2 film 7 in view of positioning between dividing region D and the overlying SiO.sub.2 film 7.
An SiO.sub.2 film has a light reflectance greater than that of a silicon nitride film. Therefore, the intensity of an incident light signal into the silicon layer in the proximity of dividing region D is reduced.
FIG. 14 shows the light reflectance of a silicon oxide film formed on a silicon layer. The thickness of the silicon oxide film (nm) is plotted along the abscissa, and light reflectance (%) is plotted along the ordinate. The wavelength X of incident light is 780 nm.
FIG. 15 is similar to FIG. 14 provided that light reflectance of a stacked portion of a silicon oxide film and a silicon nitride film is shown. The silicon nitride film has a uniform thickness of 100 nm, and the wavelength .lambda. of incident light is 780 nm.
In FIGS. 14 and 15, the silicon oxide film or the silicon nitride film is in direct contact with air. In other words, the medium of incident light is air.
The maximum light reflectance is greater than 25 % in either cases of FIGS. 14 and 15. Therefore, in split photodiodes having a conventional structure, sufficient light sensitivity can not be obtained, and it is difficult to obtain a detection signal having a sufficiently high signal-to-noise (S/N) ratio in an optical pickup.